SEROTONERGIC SPROUTING IN THE NEOSTRIATUM AFTER INTRASTRIATAL TRANSPLANTATION OF FETAL VENTRAL MESENCEPHALON

The sprouting of serotonergic fibers into the neostriatum of 6-OHDA-lesioned rats should be considered, when assessing the functional aspects of ventral mesencephalic grafts. The degree of serotonergic fiber growth into the neostriatum of rats subjected to unilateral 6-OHDA lesions followed by fetal ventral mesencephalic grafts was examined immunohistochemically. As a measure of serotonergic innervation density, the total length of immunoreactive fibers was quantified as percent area. Four weeks after transplantation, there was complete recovery of (+)-amphetamine-induced rotation. All of the ventral mesencephalic grafts contained serotonin-immunoreactive cells, but much fewer in number than the tyrosine hydroxylase-positive cells found in the same grafts. However, there was significant serotonergic hyperinnervation throughout the transplanted neostriatum compared with the control side. The hyperinnervation derived from the grafted neurons was most marked in the dorsal and lateral sectors of the rostral neostriatum. At 24 weeks after transplantation, the implanted neostriatum still showed a dense serotonergic innervation.